Abstract: Opportunistic infections are a major cause of morbidity and mortality of AIDS patients in developing countries and AIDS patients are susceptible to a number of cancers caused by opportunistic infections. Kaposi's Sarcoma (KS) is the most common tumor of AIDS patients and is the most commonly reported tumor overall in parts of Africa. The etiologic agent of KS is Kaposi's Sarcoma-associated herpesvirus (KSHV or HHV-8). KSHV is invariably found in the main KS tumor cell, the spindle cell, a cell of endothelial origin, where the virus is present predominantly in the latent state. Spindle cells express markers of lymphatic endothelial cells and appear to be most closely aligned with this endothelial cells type. While KSHV is clearly an oncogenic virus, little is known about the early steps of how KSHV infection of endothelial cells leads to tumors. It is generally recognized that one of the early steps of tumor formation is immortalization of cells. There are two main steps in immortalization of human cells, senescence and crisis. While there have been studies that have examined KSHV Induced immortalization and transformation, no clear tractable and reliable relevant human cell system has be identified. We recently found that KSHV infection of primary neonatal lymphatic endothelial cells leads to bypass of senescence, the first step in immortalization. This proposal will further analyze the KSHV mediated bypass of senescence and determine if KSHV can drive full immortalization or if other genetic steps are necessary. Both the cellular and viral mechanism of KSHV mediated immortalization of lymphatic endothelial cells will also be elucidated. These studies provide a robust system for analyzing KSHV mediated immortalization of the cell type most closely related to the main KS tumor cell, the spindle cell. A deeper understanding of how KSHV induces the earliest steps in immortalization will help identify how KSHV alters endothelial cells to become spindle cells and ultimately KS tumors and potentially provides novel therapeutic avenues for KS cell proliferation.